1. Field of Application
The present invention relates to a video signal noise reducer circuit.
2. Prior Art Technology
Noise reducer circuits are known in the prior art, for reducing the level of noise contained in a video signal, to thereby achieve improved quality of a display image obtained from that video signal. With one type of such prior art video signal noise reducer circuit, the video signal is written into a frame memory, and the average of N frames of the video signal is obtained (where N is a fixed integer). For each portion of a frame (e.g. each portion corresponding to a picture element), if there is no change from the corresponding average value (i.e. there is correlation), then that frame is transferred unchanged by the noise reducer circuit. However if there is a lack of correlation, indicating the presence of noise, then the noise power is attenuated by a factor 1/N. That is to say, the noise amplitude is attenuated by the factor 1/.sqroot.N. It is possible to use a frame memory for this purpose which is capable of storing a plurality of frames, as a non-feedback type of noise reducer circuit. However that is expensive, and an alternative is to use a feedback type of noise reducer circuit which employs a frame memory capable of storing a maximum of one frame of the video signal. This has the advantage that such a noise reducer circuit can be configured using only adders and subtractors, without multipliers being required, so that the overall circuit can be simple.
The term "frame memory" as used in the following description should be understood as being equally applicable to a 1-field memory in which successive fields of a video signal are stored (e.g. in the case of a video signal having one field per frame) and a memory in which successive frames of a video signal are stored (e.g. in the case of an interlace type of video signal having two fields per frame). It should also be noted that the descriptions of the prior art and of embodiments of the invention are directed towards processing of an input video signal which has previously been converted to digital form, i.e. consists of successive digital sample values. However for ease of understanding, the operation of the prior art examples and the subsequent embodiments of the present invention will be described based on examples analog signal waveforms.
FIG. 1 is a block diagram of a first example of a prior art feedback type of noise reducer circuit, and FIGS. 5A to 5E show examples of signal waveforms at various points within that circuit. Numeral 1 denotes a video signal input terminal, numeral 2 a video signal output terminal, 3 and 4 are subtractors, 7 denotes a feedback factor circuit, and 8 denotes a memory which can hold a fixed-duration portion of the video signal. This will be assumed to be a 1-frame memory, which thereby provides a fixed delay which is equal to one frame period. The input video signal Sb (shown in the waveform diagram of FIG. 5B) is applied to the "+" input terminals of each of the subtractors 3 and 4, while an output signal Sd produced from the feedback factor circuit 7 is applied to the "-" input terminal of the subtractor 4, to be subtracted from the input video signal Sb. An output signal Se (shown in the waveform diagram of FIG. 5E) is thereby obtained from the subtractor 4, and applied to the output terminal 2. The output signal Se is also inputted to the frame memory 8. An output signal Sa (shown in the waveform diagram of FIG. 5A) produced from the frame memory 8 is applied to the "-" input terminal of the subtractor 3, to be subtracted from the input video signal Sb, and thereby obtain a difference signal Sc (shown in the waveform diagram of FIG. 5C) from the subtractor 3. This difference signal Sc is inputted to the feedback factor circuit 7.
It will be assumed that the frame memory 8 can store one frame of the video signal. In that case, corresponding parts of two successive frames will be compared by the subtractor 3, to obtain the difference signal Sc, i.e. the instantaneous value of that signal represents a difference between two corresponding portions of the video signal which are separated along the time axis by one frame period. However the amplitude of the difference signal Sc can either be the result of noise contained in the input video signal Sb, or the result of dynamic variations in the video signal between successive frames. That is to say, the difference signal Sc will in general contain both of these respectively unwanted (i.e. noise) and wanted (i.e. actual signal variations) components.
The operation of the noise reducer circuit of FIG. 1 is based upon the assumption that any component of the difference signal Sc that results from noise contained in the input video signal Sb will be relatively small, while a component that results from actual dynamic signal variations in Sb (i.e. due to changes in the picture represented by the video signal, between successive frames) will be relatively large. For that reason, the circuit is configured such that when the absolute amplitude of the difference signal Sc is small, the feedback factor that is determined by the feedback factor circuit 7 will be large, while when the absolute amplitude of the difference signal Sc is large, the feedback factor will be made small.
FIGS. 3 and 4 respectively show two examples of input/output characteristics for the feedback factor circuit 7 which will achieve the above result.
However with the prior art circuit of FIG. 1, as is made clear by the waveform diagrams of FIGS. 5A to 5E, additional noise will be introduced into the output video signal as a result of large-amplitude changes in level of the input video signal. For example, when a large-amplitude step change occurs in the level of the input video signal as shown in FIG. 5A, then at the start of that change, a resultant step change in the level of the difference signal Sc (indicated by numeral 15 in FIG. 5C) will result in a step change in the amplitude of the feedback signal Sd (indicated by numeral 16 in FIG. 5D), and hence a corresponding spurious step change is introduced into the output video signal Se (indicated by numeral 17 in FIG. 5E). A similar phenomenon occurs as a result of each large-amplitude change of the input video signal in the opposite direction. Such a spurious amplitude change in the output video signal as that indicated by numeral 17 in FIG. 5E will result in interference in a resultant displayed video picture. For example, each vertical stripe of the desired picture will be accompanied by unwanted additional vertical stripes.
For that reason, the prior art circuit of FIG. 2 has been proposed to overcome the above disadvantage of the circuit of FIG. 1. This differs from the prior art circuit of FIG. 1 only in the addition of a high pass filter (HPF) 5 which is connected between the output of the subtractor 3 and the feedback factor circuit 7. Essentially, the circuit of FIG. 2 is based on the fact that relatively low-frequency components of the difference signal Sc will, statistically, represent actual dynamic components of the input video signal, rather than noise components. Relatively high-frequency components of the difference signal Sc on the other hand will generally consist of noise components. The output signal from the subtractor 3 is therefore passed through the HPF 5 before being supplied to the feedback factor circuit 7.
FIGS. 6A to 6E shows waveform diagrams of output signal Sa from the frame memory 8, the input video signal Sb, and output signals Sc, Sd and Se from the sb3, the feedback factor circuit 7, and the subtractor 4 of the prior art noise reducer circuit of FIG. 2. The basic operation of the circuit of FIG. 2 is essentially identical to that of FIG. 1, so that detailed description will be omitted. With the noise reducer circuit of FIG. 2, the high frequency components of the difference signal Sc from the subtractor 3 are extracted by the HPF 5 and supplied to the feedback factor circuit 7, i.e. as the noise component of the difference signal Sc.
The circuit of FIG. 2 suppresses low frequency components of the signal Sd that is produced from the feedback factor circuit 7. However as shown in FIG. 6D, a transient signal voltage occurs (e.g. as indicated by numeral 20 in FIG. 6D) in response to each edge of an abrupt large-amplitude change in the input video signal Sb, with each transient consisting of two successive relatively large-amplitude peaks of opposite polarity. As a result, corresponding transients (e.g. as indicated by numeral 22 in FIG. 6E) will be produced in the output video signal Se from the subtractor 4. Thus, noise interference will also be generated in a displayed picture obtained from the output video signal Se of the prior art noise reducer circuit of FIG. 2, i.e. interference that is generated as a result of operation of the noise reducer circuit itself.